1. Technical Field
This invention relates to a thin-film EL device having at least a structure comprising an electrically insulating substrate, a patterned electrode layer on the substrate, and a dielectric layer, a light-emitting layer and a transparent electrode layer stacked on the electrode layer.
2. Background Art
EL devices are now practically used in the form of backlights for liquid crystal displays (LCDs) and watches. The EL devices work on a phenomenon in which a substance emits light at an applied electric field, viz., an electro-luminescence (EL) phenomenon.
The EL devices are divided into two types: dispersion type EL devices having a structure wherein electrode layers are provided on the upper and lower sides of a dispersion of light-emitting powder in an organic material or porcelain enamel, and thin-film EL devices having a thin-film light-emitting substance sandwiched between two electrode layers and two thin-film insulators on an electrically insulating substrate. These types of EL devices are each driven in a direct or alternating voltage drive mode. Known for long, the dispersion type EL device has the advantage of ease of fabrication; however, it has only limited use on account of low luminance and short service life. On the other hand, the thin-film EL device has recently wide applications due to the advantages of high luminance and a long lifetime.
FIG. 2 shows the structure of a double-insulation type thin-film EL device typical of prior art thin-film EL devices. This thin-film EL device includes a transparent substrate 21 formed of a green glass sheet used for liquid crystal displays or PDPs, and a transparent electrode layer 22 formed of ITO or the like to a thickness of about 0.2 xcexcm to 1 xcexcm in a predetermined stripe pattern, a first insulator layer 23 in transparent thin-film form, a light-emitting layer 24 having a thickness of about 0.2 xcexcm to 1 xcexcm and a second insulator layer 25 in transparent thin-film form stacked on the substrate. Further, an electrode layer 26 is formed by patterning an Al thin-film or the like in stripes extending perpendicular to the transparent electrode layer 22. The transparent electrode layer 22 and the electrode layer 26 together define a matrix, in which voltage is selectively applied to a selected area of light-emitting substance to allow the light-emitting substance of that specific pixel to emit light. The resultant light is extracted from the substrate side. Having a function of limiting current flow through the light-emitting layer, the thin-film insulator layers are able to inhibit the dielectric breakdown of the thin-film EL device, and contribute to the achievement of stable light-emitting properties. Thus, the thin-film EL device of this structure now finds wide commercial applications.
For the aforesaid thin-film transparent insulator layers 23 and 25, transparent dielectric thin films of Y2O3, Ta2O5, Al3N4, BaTiO3, etc. are formed to a thickness of about 0.1 to 1 xcexcm by sputtering, evaporation or the like.
Among light-emitting materials, Mn-doped ZnS exhibiting yellowish orange light emission has mainly been used for ease of film formation and light-emitting properties. For color display fabrication, the use of light-emitting materials capable of emitting light in the three primary colors, red, green and blue is inevitable. These materials known so far in the art, for instance, include Ce-doped SrS and Tm-doped ZnS exhibiting blue light emission, Sm-doped ZnS and Eu-doped CaS exhibiting red light emission, and Tb-doped ZnS and Ce-doped CaS exhibiting green light emission.
The light-emitting materials disclosed in Shosaku Tanaka, xe2x80x9cthe Latest Development in Displaysxe2x80x9d in Monthly Display, April, 1998, pp. 1-10, include ZnS, Mn/CdSSe, etc. as red light-emitting materials, ZnS:TbOF, ZnS:Tb, etc. as green light-emitting materials, and SrS:Cr, (SrS:Ce/ZnS)n, Ca2Ga2S4:Ce, Sr2Ga2S4:Ce, etc. as blue light-emitting materials. Such light-emitting materials as SrS:Ce/ZnS:Mn are also disclosed as white light-emitting materials.
International Display Workshop (IDW), 1997, X. Wu, xe2x80x9cMulticolor Thin-Film Ceramic Hybrid EL Displaysxe2x80x9d, pp. 593-596 discloses that among the aforesaid materials, SrS:Ce is used in a thin-film EL device having a blue light-emitting layer. In addition, this article discloses that when a light-emitting layer of SrS:Ce is formed, an electron beam evaporation process in a H2S atmosphere enables to form a light-emitting layer of high purity.
However, for these thin-film EL devices, a structural problem remains unsolved. The problem is that since the insulator layers are each formed of a thin film, it is difficult to reduce to nil steps at the edges of the pattern of the transparent electrode, which occur when a large area display is fabricated, and defects in the thin-film insulators, which are caused by dust, etc. occurring in the production process, resulting in a destruction of the light-emitting layer due to a local dielectric strength drop. Such defects offer a fatal problem to display devices, and become a bottleneck in the wide practical use of thin-film EL devices in a large-area display system, in contrast to liquid crystal displays or plasma displays.
To provide a solution to the defect problem with such thin-film insulators, JP-A07-50197 and JP-B07-44072 disclose a thin-film EL device using an electrically insulating ceramic substrate as a substrate and a thick-film dielectric material instead of the thin-film insulator located beneath the light-emitting substance. As shown in FIG. 3, this thin-film EL device has a structure having a lower thick-film electrode layer 32, a thick-film dielectric layer 33, a light-emitting layer 34, a thin-film insulator layer 35 and an upper transparent electrode 36 stacked on a substrate 31 such as a ceramic substrate. Unlike the thin-film EL device shown in FIG. 2, the transparent electrode layer is formed at the top of the device because the light emitted from the light-emitting substance is extracted out of the upper side of the device facing away from the substrate.
The thick-film dielectric layer in this thin-film EL device has a thickness of several tens of nanometers to several hundreds of microns that is several hundred to several thousand times as thick as the thin-film insulator layer. Thus, the thin-film EL device has the advantages of high reliability and high fabrication yields because of little dielectric breakdown caused by pinholes formed by steps at electrode edges or dust, etc. occurring in the device fabrication process. Although the use of this thick- film dielectric layer leads to a problem that the effective voltage applied to the light-emitting layer drops, this problem can be solved or eliminated by using a high permittivity material for the dielectric layer.
However, the light-emitting layer formed on the thick-film dielectric layer has a thickness of barely several hundreds of nanometers that is about {fraction (1/100)} of that of the thick-film dielectric layer. For this reason, the thick-film dielectric layer must have a smooth surface at a level less than the thickness of the light-emitting layer. However, it is still difficult to sufficiently smooth down the surface of a dielectric layer fabricated by an ordinary thick-film process.
To be more specific, a thick-film dielectric layer, because of being essentially constructed of ceramics using a powdery material, usually suffers a volume shrinkage of about 30 to 40% upon dense sintering. However, ordinary ceramics are consolidated through a three-dimensional shrinkage upon sintering whereas a thick-film ceramic material formed on a substrate does not shrink across the substrate because the thick film is constrained to the substrate; its volume shrinkage occurs only in the thickness direction or one-dimensionally. For this reason, the sintering of the thick-film dielectric layer does not proceed to a sufficient extent, yielding an essentially porous layer.
Since the consolidation process proceeds through a solid phase reaction of ceramic powder having a certain particle size distribution, abnormally sintered sites such as abnormal crystal grain growth and macropores are likely to occur. In addition, the surface roughness of the thick film is absolutely greater than the crystal grain size of polycrystalline sintered body, and accordingly, the thick film has surface asperities of at least submicron size even in the absence of such defects as mentioned above.
When the dielectric layer has surface defects or a porous structure or asperity shape as mentioned above, it is impossible to deposit thereon a uniform light-emitting layer by evaporation, sputtering or the like because the light-emitting layer is conformal to the surface shape of the dielectric layer. This results in problems such as a decrease in effective light-emitting area because an electric field cannot be effectively applied to the portions of the light-emitting layer formed on non-flat portions of the substrate, and a decrease in light emission luminance because local non-uniformity of thickness causes a local dielectric breakdown of the light-emitting layer. Furthermore, locally large thickness fluctuations cause the strength of an electric field applied to the light-emitting layer to locally vary too largely to obtain any definite light emission voltage threshold.
Thus, conventional fabrication processes needed operations of polishing down large surface asperities of a thick-film dielectric layer and then removing finer asperities by a sol-gel step.
However, the polishing of a large-area substrate for display or other purposes is technically difficult to achieve, and is a factor for cost increases. The addition of the sol-gel step is another factor for cost increases. When a thick-film dielectric layer has abnormally sintered sites which may give rise to asperities too large for removal by polishing, they cannot be removed even by the addition of the sol-gel step, which causes a drop of manufacturing yield. It is thus very difficult to use a thick-film dielectric material to form a light emission defect-free dielectric layer at low cost.
A thick-film dielectric layer is formed by a ceramic powder material sintering process where a high firing temperature is needed. As is the case with ordinary ceramics, a firing temperature of at least 800xc2x0 C. and usually 850xc2x0 C. is needed. To obtain a dense thick-film sintered body in particular, a firing temperature of at least 900xc2x0 C. is needed. In consideration of heat resistance and a reactivity problem with respect to the dielectric layer, the substrate used for the formation of such a thick-film dielectric layer is limited to alumina or zirconia ceramic substrate; it is difficult to rely on inexpensive glass substrates. The requisite for the ceramic substrate to be used for display purposes is that it has a large area and satisfactory smoothness. The substrate meeting such conditions is obtained only with much technical difficulty, and is yet another factor for cost increases.
For the metal film used as the lower electrode layer, its heat resistance requires to use expensive noble metals such as palladium and platinum. This, too, is a factor for cost increases.
In order to solve such problems, the inventor proposed in Japanese Patent Application No. 2000-299352 to form a multilayer dielectric layer thicker than a conventional thin-film dielectric layer, by repeating the solution coating-and-firing process plural times, for use in place of a conventional thick-film dielectric material or a thin-film dielectric material formed by a sputtering process or the like.
FIG. 4 shows the structure of a thin-film EL device using the aforesaid multilayer dielectric layer. In this thin-film EL device, a lower electrode layer 42 having a predetermined pattern is stacked on an electrically insulating substrate 41. A multilayer dielectric layer 43 is formed on the lower electrode layer by repeating the solution coating-and-firing process plural times. Further a light-emitting layer 44 and preferably a thin-film insulator layer 45 and a transparent electrode layer 46 are stacked on the dielectric layer.
As compared with a conventional thin-film dielectric layer, the multilayer dielectric layer having such structure is characterized in that high dielectric strength is achievable, locally defective insulation due to dust or the like occurring during the process is effectively prevented, and much improved surface flatness is obtainable. For a thin-film EL device using the aforesaid multilayer dielectric layer, glass substrates less expensive than ceramic substrates may be used because the dielectric layer can be formed at a temperature lower than 700xc2x0 C.
However, when the multilayer dielectric layer is formed by a solution coating-and-firing process, using a lead-based dielectric material as the dielectric layer material, a light-emitting layer to be formed on the dielectric layer can react with the lead component of the dielectric layer, giving rise to some practically unfavorable problems such as initial light emission luminance drops, luminance variations, and changes of light emission luminance with time
An object of the present invention is to provide, without incurring any cost increase, a thin-film EL device which eliminates any restriction on the selection of substratesxe2x80x94that is one problem associated with a conventional thin-film EL devicexe2x80x94so that glass and similar substrates which are inexpensive and easy to form to a large area can be used, and enables non-flat portions of a dielectric layer due to an electrode layer or dust or the like during processing to be corrected by a quick-and-easy process and the dielectric layer to have improved surface flatness. Especially when the invention is applied to a thin-film EL device having a multilayer dielectric layer formed using a lead-based dielectric material as mentioned above, high display qualities can be obtained with no light emission luminance drop, no luminance variation, and no change of light emission luminance with time. Another object of the present invention is to provide a process for fabricating the thin-film EL device.
The above objects are achieved by the following embodiments of the invention.
(1) A thin-film EL device having at least a structure comprising an electrically insulating substrate, a patterned lower electrode layer stacked on said substrate, and a dielectric layer, a light-emitting layer and an upper electrode layer stacked on said lower electrode layer, at least one of said lower electrode and said upper electrode being a transparent electrode, wherein
said dielectric layer has a multilayer structure wherein lead-based dielectric layers formed by repeating a solution coating-and-firing process plural times and a non-lead-based, high-permittivity dielectric layer are stacked, and
an uppermost surface layer of said dielectric layer having a multilayer structure is the non-lead-based, high-permittivity dielectric layer.
(2) The thin-film EL device of (1), wherein said lead-based dielectric layer has a thickness of 4 xcexcm to 16 xcexcm inclusive.
(3) The thin-film EL device of (1), wherein said non-lead-based dielectric layer has a thickness of more than 0.2 xcexcm.
(4) The thin-film EL device of any one of (1) to (3), wherein said non-lead-based, high-permittivity dielectric layer is made of a perovskite structure dielectric material.
(5) The thin-film EL device of any one of (1) to (4), wherein said non-lead-based, high-permittivity dielectric layer is formed by a sputtering process.
(6) The thin-film EL device of any one of (1) to (4), wherein said non-lead-based, high-permittivity dielectric layer is formed by the solution coating-and-firing process.
(7) The thin-film EL device of (6), wherein said dielectric layer having a multilayer structure is formed by repeating the solution coating-and-firing process at least three times.
(8) A process for fabricating a thin-film EL device having at least a structure comprising an electrically insulating substrate, a patterned lower electrode layer stacked on said substrate, and a dielectric layer, a light-emitting layer and an upper electrode layer stacked on said lower electrode layer, at least one of said lower electrode and said upper electrode being a transparent electrode, said process comprising the step of:
stacking lead-based dielectric layers formed by repeating a solution coating-and-firing process plural times and a non-lead-based, high-permittivity dielectric layer to form a multilayer structure such that an uppermost surface layer of the dielectric layer having the multilayer structure is the non-lead-based, high-permittivity dielectric layer.
(9) The thin-film EL device fabrication process of (8), wherein said non-lead-based, high-permittivity dielectric layer is formed by a sputtering process.
(10) The thin-film EL device fabrication process of (8), wherein said non-lead-based, high-permittivity dielectric layer is formed by the solution coating-and-firing process.
(11) The thin-film EL device fabrication process of (10), wherein said dielectric layer having the multilayer structure is formed by repeating the solution coating-and-firing process at least three times.